SIMULATION OF VARIABLE-DENSITY FLOW AND TRANSPORT OF REACTIVE AND NONREACTIVE SOLUTES DURING A TRACER TEST AT CAPE-COD, MASSACHUSETTS

A multispecies numerical code was developed to simulate flow and mass transport with kinetic adsorption in variable-density flow systems. Th e two-dimensional code simulated the transport of bromide (Br-), a non reactive tracer, and lithium (Li+), a reactive tracer, in a large-scal e tracer test performed in a sand-and-gravel aquifer at Cape God, Mass achusetts. A two-fraction kinetic adsorption model was implemented to simulate the interaction of Li+ with the aquifer solids. Initial estim ates for some of the transport parameters were obtained from a nonline ar least squares curve-fitting procedure, where the breakthrough curve s from column experiments were matched with one-dimensional theoretica l models, The numerical code successfully simulated the basic characte ristics of the two plumes in the tracer test. At early times the cente rs of mass of Br- and Li+ sank because the two plumes were closely cou pled to the density-driven velocity field, At later times the rate of downward movement in the Br- -plume due to gravity slowed significantl y because of dilution by dispersion, The downward movement of the Liplume was negligible because the two plumes moved in locally different velocity regimes, where Li+ transport was retarded relative to Br-. T he maximum extent of downward transport of the Li+ plume was less than that of the Br- plume. This study also found that at early times the downward movement of a plume created by a three-dimensional source cou ld be much more extensive than the case with a two-dimensional source having the same cross-sectional area, The observed shape of the Br- pl ume at Cape Cod was simulated by adding two layers with different hydr aulic conductivities at shallow depth across the region, The large dis persion and asymmetrical shape of the Li+ plume were simulated by incl uding kinetic adsorption-desorption reactions.